We are developing a library of monoclonal antibodies that are directed against a cell surface glycopeptide, isolated from bovine cerebral cortex cells, that apparently is related to density-dependent growth regulation. The monoclonal antibodies will be indispensable to: (1)\purify the cell surface glycopeptide; (2)\use as probes to follow the fate of the glycopeptide inhibitor subsequent to its interaction with target cells; (3)\screen both normal and transformed cells for the presence of similar antigenic substances on their cell surfaces; (4)\isolate and purify the parental glycoprotein from solubilized plasma membranes; and (5)\compare the bovine glycopeptide inhibitor to growth-regulating substances isolated in other laboratories. Both in vivo and in vitro immunization procedures are being used to elicit monoclonal antibodies to the inhibitor. In vivo procedures have provided us with two monoclonal IgG antibodies that have been used to demonstrate that inhibitory glycopeptides obtained from mouse and bovine cerebral cortex cells are antigenically related and that the glycopeptides originated from the cerebral cortex cell surface. In addition, these monoclonal antibodies have been used to follow the inhibitor preparations in radioimmune assays and to precipitate the biological activity when reacted with the glycopeptide in the presence of Staph A protein. Although the inhibitor has been shown to bind to affinity columns prepared with these two monoclonal antibodies, the glycopeptide can be removed only by conditions that denature the molecule. The development of in vitro immunization procedures, where IgM with lower affinity constants most likely would be obtained, should be a major step forward towards our goal. (A)